Conventionally, for purpose of improvement of catalytic conversion efficiency of a catalyst used for purification of exhaust gas, an emission control system for an internal combustion engine includes exhaust-gas sensors (e.g., an air/fuel sensor and an oxygen sensor) that are respectively disposed upstream and downstream of the catalyst in a flow direction of the exhaust gas. The exhaust-gas sensors detect an air-fuel ratio of the exhaust gas or detects whether the exhaust gas is rich or lean.
When the air-fuel ratio of the exhaust gas changes from rich to lean or from lean to rich, an output change of the exhaust-gas sensor, such as an oxygen sensor, may lag behind a change of an actual air-fuel ratio of the exhaust gas. Thus, the exhaust-gas sensor may have a room for improvement in detection responsiveness.
For example, as described in Patent Document 1 (JP 8-20414 corresponding to U.S. Pat. No. 4,741,817), at least one of an auxiliary electrochemical cell is incorporated to an inside of a gas sensor such as an oxygen sensor for increase in detection responsiveness.
As described in Patent Document 2 (JP 2000-054826 A), a catalyst such as a three-way catalyst used for purification of exhaust gas may possibly be in a lean state, in which an oxygen amount stored in the catalyst (i.e., an oxygen amount adsorbed in the catalyst) is relatively large, after a fuelling-stop control in which fuel injection of an internal combustion engine is stopped, i.e., after resumption of the fuel injection. Thus, a rich direction control is performed after the fuelling-stop control in an emission control device in Patent Document 2, in which an air-fuel ratio of exhaust gas is controlled to be richer. Accordingly, it can be limited that the catalyst becomes in the lean state, in other words, the oxygen amount stored in the catalyst can be reduced.
In Patent Document 1, the auxiliary electrochemical cell is necessarily incorporated to the inside of the gas sensor. Thus, when the auxiliary electrochemical cell is incorporated to a general gas sensor that does not have an auxiliary electrochemical cell, the general gas sensor may need to be changed greatly in structure. For practical use, the gas sensor may be required to be changed in design, and a manufacturing cost of the gas sensor may be increased.
The present inventors study a system that changes an output characteristic of an exhaust-gas sensor located downstream of a catalyst so as to increase responsiveness (lean responsiveness) of the exhaust-gas sensor with respect to lean gas, in order to detect a decrease of NOx conversion efficiency of the catalyst. In the emission control device in Patent Document 2, the rich direction control may be terminated when the output of the exhaust-gas sensor located downstream of the catalyst exceeds a rich threshold. In this case, if a control for increase of the lean responsiveness of the exhaust-gas sensor is performed during the rich direction control, a time point when the output of the exhaust-gas sensor exceeds the rich threshold may retard because responsiveness (rich responsiveness) of the exhaust-gas sensor with respect to rich gas is relatively low during the control for increase of the lean responsiveness. Therefore, the termination of the rich direction control may retard, and emission rate of CO, HC may be thereby increased. As a result, emission gas may be deteriorated.
It is an objective of the present disclosure to provide an emission control system for an internal combustion engine, which is capable of changing an output characteristic of an exhaust-gas sensor without great change in design and cost increase, and capable of limiting deterioration of emission gas due to a rich direction control performed after a fuelling-stop control.